The present invention relates to an aggregate for producing a fresh air stream.
In order to increase the action of controlled three-way catalysts it is known to use two methods, namely an exhaust gas return and a secondary air supply. These methods minimize the production of nitrogen oxides or reduce the contents of carbon monoxide and hydrocarbons before the exhaust gas reaches the catalyst. During the secondary air supply the fresh air is supplied to the exhaust gas line as short as possible behind the motor, for eliminating hydrocarbon compounds and carbon monoxide produced during the combustion process. Therefore a post-combustion of the exhaust gas is provided at temperatures of over 600.degree. C. During this additional combustion process heat is supplied to the catalyst. This is especially important during a so-called cold start in which a relatively reach mixture is provided an automatically the CO-and HC-fractions are increased in an overproportional degree. The secondary air supply in the hot exhaust gas practically triggers a post-combustion, during which the carbon monoxide and hydrocarbons which are not burned in the motor, are post-oxidized or post-burnt. The secondary air supply however has a further effect. The controlled catalyst operates first to a predetermined operational temperature optimally. This warm-up phase can last ample time during which the motor vehicle covers several kilometers. The post-combustion by a secondary air supply system increases the exhaust gas temperatures especially in this warm-up phase. Higher temperatures provide faster response of the G-catalyst. The G-catalyst can therefore earlier perform its objective to convert the contaminants.
The present invention is a further improvement of the aggregate disclosed in the German document DE-OS 41 15 805. In this aggregate a spring loaded check valve is arranged in the fresh air supply so as to prevent a return flow of the exhaust gas into the suction region of the internal combustion engine when the pressure in the exhaust gas conduit exceeds the feeding pressure of the blower. Thereby predetermined operational conditions can be produced in the exhaust gas conduit which lead to fluttering of the check valve, so that a return flow of the exhaust gas cannot be excluded with the required reliability.